Why Runoff Volume?

2/25/2014ESTIMATING RUNOFF VOLUME ROCKET SCIENCEBy: Matthew B. Poling, P.E.matt.poling@ncdenr.govWhy Runoff Volume? Many stormwater programs have avolume control requirement; that is,capturing the first 1 or 1.5 inches ofstormwater and retaining it for 2 to 5days. Similarly, Erosion Control measuresare required to capture a volume anddischarge it over a 2 to 5 day period.1

2/25/2014Hydrologic Components Watershed Characteristics – Drainage Area,Land Use, Types of Soil, and Time ofConcentration. Storm Characteristics – Type, Duration,Total Volume, Intensity, and Distribution.SELECTING THE RIGHT METHOD! Acceptable methods to calculate peak discharge.1. Rational Method2. NRCS,TR-55 Graphical Peak Flow Method(alsoknown as the SCS Method) Which method do I use?1. Rational Method: drainage area is less than 50 acres2. SCS Method: drainage area is greater than 20 acres, alsothe SCS method should only be used when the CurveNumber exceeds 50 and the time of concentration isgreater than 0.1 hr and less than 10 hr.2

2/25/2014USING THE NRCS METHOD Step 1. Determine the drainage area. Step 2. Determine a weighted Curve Number and Tc Step 3. Select appropriate Rainfall amounts. (Depth, notintensity) Step 4. Determine peak discharge.Example #1 Using the SCS Method, determine the total amount of runoffvolume produced from a 10 year storm event that is located inBoone, NC. Two-thirds of the site is to be cleared and graded,while one-third will be left alone as dense woods. Drainage area of 45 acres Assume all soils are Hydrologic Group B The time of concentration: Sheet flow of 50’ with a slope of 8% made of Dense Woods. Shallow concentrated flow of 250’ with a slope of 6.5% unpaved. Channel flow of 450’ with a slope of 3.5% using a bankfull flow areaof 4.5 ft2 and a wetted perimeter of 5.0’. (Use n 0.055) m3

2/25/2014Hydrologic ComponentsNRCS SOIL WEB SURVEY4

2/25/2014NRCS SOIL WEB SURVEYNRCS SOIL WEB SURVEY5

2/25/2014Four Hydrologic Soil Groups asDefined by the SCS (1986)Group A - A soils have low runoff potential and high infiltration rates and have a high rate ofwater transmission (greater than 0.30 in/hr). The textures of these soils are typically sand,loamy sand, or sandy loam.Group B - B soils have moderate infiltration rates and have a moderate rate of watertransmission (0.15-0.30 in/hr). The textures of these soils are typically silt loam or loam.Group C - C soils have low infiltration rates and have a low rate of water transmission (0.050.15 in/hr). The texture of these soils is typically sandy clay loam.Group D - D soils have high runoff potential and have a very low rate of water transmission(0-0.05 in/hr). The textures of these soils are typically clay loam, silty clay loam, sandy clay,silty clay, or clay.NRCS SOIL WEB SURVEY6

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2/25/2014LAND USE Total Drainage area 45 acres (2/3) * 45 30 acres cleared (1/3) * 45 15 acres dense woods CN Values (These values can be found in the Manual, Chapter 8.) CN 86 for Newly Graded Areas CN 55 for Dense Woods Weighted CN-Value CN*A (86)*(30 ac) (55)*(15 ac) 3405 (Divide by total Acreage) Weighted CN-Value CN*A/AT 3405/45 75.67TR-55The curve number, CN, describes the characteristics of the drainage area thatdetermines the amount of runoff generated by a given storm: hydrologic soil groupand groundcover. Soils are classified into four hydrologic soil groups (A, B, C, andD) based on their minimum infiltration rate, with A having the highest infiltrationpotential and D having the lowest.8

2/25/2014Hydrologic Components Time of concentration is a concept used inhydrology to measure the response of awatershed to a rain event. It is defined as thetime needed for water to flow from the mostremote point in a watershed to the watershedoutlet. It is a function of the topography,geology, and land use within the watershed.(Wikipedia) tc tsheet tshallow tchannel9

2/25/2014Time of Concentration10

2/25/2014TIME OF CONCENTRATIONTR-55 13.14 MinutesRAINFALL11

2/25/2014SCS Peak Discharge Method Q (P-0.2S)2(P 0.8S) Q Depth of Runoff (in) over the entire watershed. P Rainfall (Depth in inches of a 24 hour event) S Potential maximum retention after run off begins (in) S 1000 -10; S 3.22CN Q (6.61 – (0.2*3.22))2 ; Q 3.88 Inches of Runoff(6.61 (0.8*3.22)Graphical Peak Discharge Method12

2/25/2014SCS Peak Discharge Method Peak Discharge (Qp) qu * Am * Fp * Q CFS Unit Peak Discharge (qu) From figure 8.03k in the sedimentmanual (CFS / Square Mile per Inch of Rainfall) Drainage Area (Am) mi2 Pond and Swamp Factor (Fp) 1.0 0% Runoff (Q) Depth in inches13

2/25/2014INITIAL ABSTRACTIONCN 75.67 Average (Ia) 0.65”Initial abstraction (Ia) is all losses beforerunoff begins. It includes water retained insurface depressions, water being interceptedby vegetation, evaporation, and infiltration. Ia/P 0.65 / 6.61 0.1Unit Hydrograph14

2/25/2014UNIT PEAK DISCHARGEqu 750 csm/in.Ia/P 0.1Tc 0.219SCS Peak Discharge Method Peak Discharge (Qp) qu * Am * Fp * Q CFS Unit Peak Discharge (qu) 350 csm/in (figure 8.03k in thesediment manual) (CFS / Square Mile per Inch of Rainfall) Drainage Area (Am) 45 Acres/640 ac/mi2 0.07 mi2 Pond and Swamp Factor (Fp) 1.0 0% Runoff (Q) 3.88 inches Qp 750 * 0.07 * 1.0 * 3.88 204 cfs15

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2/25/2014RUNOFF VOLUME Q 3.88 Inches of Runoff Area 45 Acres Volume Depth x Area Volume 627,264 Cubic Feet of Runoff Volume 14.4 ac-ft17

2/25/2014EXAMPLE #2 Using the Rational Method in conjunction with the SCSTriangular Unit Hydrograph Method, determine the totalamount of runoff volume produced from a 10 year storm event.Two-thirds of the site is to be cleared and graded, while onethird will be left alone as dense woods. Drainage area of 45 acres Weighted Runoff Coefficient The time of concentration: Sheet flow of 50’ with a slope of 8% made of Dense Woods. Shallow concentrated flow of 250’ with a slope of 6.5% unpaved. Channel flow of 450’ with a slope of 3.5% using a bankfull flow areaof 4.5 ft2 and a wetted perimeter of 5.0’. (Use n 0.055)RATIONAL METHOD Step 1. Determine the Land Use “C” Values. Step 2. Determine your time of concentration, time to peak,and time of base. Step 3. Select Appropriate Storm18

2/25/2014LAND USE Total Drainage area 45 acres (2/3) * 45 30 acres cleared (1/3) * 45 15 acres dense woods C Values (These values can be found in the Manual, Chapter 8.) C 0.6 for Newly Graded Areas C 0.25 for Dense Woods Weighted CN-Value CN*A (0.6)*(30 ac) (0.25)*(15 ac) 21.8 (Divide by total Acreage) Weighted CN-Value CN*A/AT 21.8/45 0.4819

2/25/2014Time of Concentration and the Jarrett Method Jarrett (2005) determined that the time ofconcentration, tc, is approximated as 5minutes for watersheds smaller than theJarrett Maximum Area.Jarrett Method for Small Drainage Areas Ajarrett Jarrett Maximum Area (ac) Ajarrett 460 * (S) S H / L S Average Slope Length (ft) H Change in Elevation (ft) L Flow Length (ft)20

2/25/2014Jarrett Method for Small Drainage Areas Drainage Area 45 acres Ajarrett 460 * (S) S 0.048 ft/ft Ajarrett 460 * (0.048) 22.08 45 22.08 Therefore cannot assume 5minute time of concentrationTIME OF CONCENTRATIONThe Kinematic Wave TheorySOLVED THROUGH ITERATIONS21

2/25/2014STORM DURATIONBy setting the storm duration equal to the time of concentration, you areproducing a storm event with the maximum peak discharge.Q: Why did you set the storm duration greater than the time ofconcentration?A: You want to make sure that all of the drainage area is contributing.Tc 15.5 min.22

MATING PEAK FLOW RATIONAL EQUATION, Q CIA C 0.48 I 5.06 in/hr A 45 Acres Q 109 cfs23

2/25/2014SCS TRIANGULARHYDROGRAPH METHOD Step 1. Solve for Peak Flow and time to peak Step 2. Apply values to the SCS Triangular unit Hydrograph Step 3. Find the area of the triangle! (.5 * Base * Heights)SCS TRIANGULAR HYDROGRAPH24

2/25/2014Time to Peak, Time of Base Lag Time (TL) TC * 0.6 (SCS Empirical Equation) Time to Peak (TP) D/2 TL TP D/2 TC * 0.6 TP (15/2) (15.5 * 0.6) 16.8 Minutes Time of Base (TR) 1.67 * TP TR 1.67 * 16.8 28 MinutesAREA UNDER THE TRIANGLE25

2/25/2014RUNOFF VOLUME Solving for the area under the triangles!(0.5*106*16.8*60) (0.5*106*28*60)53,424 89,040 142,464 Cubic Feet of Runoff!Also, Volume 3.3 Acre-FeetWHY IS THE VOLUME SO DIFFERENT? Volume 24 hr event 14.4 Acre-Feet Volume 15 min. event 3.3 Acre-Feet Peak Flow 24 hr event 204 Peak Flow 15 min. event 106From a depth perspective!26

2/25/2014Why Runoff Volume? Sediment traps and basins are using total runoff volume andpeak discharge rate. Designing with Peak discharge does not account for increasedamounts of volume. This leads to traps and basins being overtopped! Is a type II storm distribution unreasonable for sediment trapand basin design?27

USING THE NRCS METHOD Step 1. Determine the drainage area. Step 2. Determine a weighted Curve Number and Tc Step 3. Select appropriate Rainfall amounts. (Depth, not intensity) Step 4. Determine peak discharge. Example #1 Using the SCS Method, determine the total amount of runoff volume produced from a 10 year storm event that is located in